US President George W Bush is calling for a "democratic transition" in Cuba, eventually culminating with free and fair elections, after Cuban President Fidel Castro steps down.

The world's longest serving political leader, Mr Castro announced he was stepping down in the online edition of official Communist Party newspaper Granma.

"I believe that the change from Fidel Castro ought to begin a period of democratic transition," said Mr Bush.

He signalled no change in a half-century of tough US policies towards America's socialist one-party neighbour.

The US president said the "first step" was for the Cuban Government to release political prisoners, and urged the international community to help build democratic institutions in Cuba.

"We're going to help. The United States will help the people of Cuba realise the blessings of liberty," he said.

Mr Castro did not say who he thought should be his successor as president, though most analysts believe his brother Raul Castro, 76, is the obvious choice.

"There will be an interesting debate that will arise, eventually. There will be some who say 'let's promote stability'," said Mr Bush.

"Of course, in the meantime, political prisoners will rot in prison and the human condition will remain pathetic in many cases.

Sunday decision

Cuba's newly-elected parliament is due to meet for the first time this Sunday to elect a new executive Council of State, whose president becomes president of Cuba.

Fidel Castro has been president in name only since July 2006, when he temporarily handed over power to his younger brother Raul because of ill health.

He has now ruled out resuming his role.

Under the headline "Message from the Commander in Chief", the 81-year-old revolutionary leader wrote: "I neither aspire nor will I accept the position of President of Council of State and Commander in Chief."

Mr Castro has ruled Cuba since 1959, when he seized power in a communist revolution.

He insists he is not saying farewell to public life, vowing to continue writing essays for the state newspaper.